1. Field of Invention
The invention concerns high velocity metal forming (HVMF). More particularly, the invention relates to a magnetic coil assembly (actuator) for use in electromagnetic forming as a reliable alternative to traditional metal-stamping or metal-forming operations. A method of using the actuator is also contemplated.
2. Description of Related Art
Electromagnetic forming is a method of forming sheet metal or thin walled tubes that is based on the method of placing a work-coil in close proximity to the metal to be formed and running a brief, high intensity current pulse through the coil. If the metal to be formed is sufficiently conductive the change in magnetic field produced by the coil will develop eddy currents in the workpiece. These currents also have associated with them a magnetic field that is repulsive to that of the coil. This natural electromagnetic repulsion is capable of producing very large pressures that can accelerate the workpiece at high velocities (typically 50-500 meters/second). This acceleration is produced without making physical contact with the workpiece. The electrical current pulse is usually generated by the discharge of a capacitor bank. It can provide: improved formability, improved strain distribution, reduction in wrinkling, active control of springback and the possibility of local coining and embossing.
Electromagnetic forming can be carried out on a wide range of materials and geometries within some fundamental constraints. First, the material must be sufficiently electrically conductive to exclude the electromagnetic field of the work-coil. The physics of this interaction have been well characterized.
The efficiency of electromagnetic forming is directly related to the resistance of the workpiece material. Materials which are poor conductors can only be effectively formed with electromagnetic energy if an auxiliary driver plate of high conductivity is used to push the workpiece.
Electromagnetic forming of axis-symmetric parts, using either compression or expansion solenoid type forming coils is presently the most widely used of the electric pulse energy methods. The common application is for the swaging of tubular components onto coaxial mating parts for assembly. Not as common is non-symmetric forming that concerns the forming of shell or dish shapes within a forming die using workpieces comprising flat sheets of metal.
If conventional electromagnetic forming coils are used in non-symmetric forming, the electromagnetic pressure distribution must be appropriate for the part being formed. It has been found that the velocity distribution within the sheet metal during forming significantly influences the result. Puckers or other defects can form when the launch velocity of the metal workpiece is not uniform.
In principle, as shown in FIG. 1, an axis-symmetric electromagnetic forming system consists of a capacitor bank 1, a conductive actuator 2 and the metallic workpiece 3 to be deformed, and the forming die 4 that has a die cavity 5 provided in one of its surfaces.
The capacitor bank 1 is connected to the actuator 2, which is located near the workpiece 3 and the die 4. When the main switch is closed, the large current through the actuator 2 produces a transient magnetic field that induces eddy currents in the nearby metallic workpiece 3. The currents in the actuator 2 and the metallic workpiece 3 travel in opposite directions, according to Lenz's Law. The electromagnetic repulsion between the oppositely flowing currents, governed by the Lorentz force, provides the deformation force to the workpiece 3, forcing it against the surface of the die 4 such that the workpiece assumes the shape of the die cavity 5 thereby providing a formed part.
High velocity forming methods have had a recent resurgence in interest due to the need for greater use of aluminum alloys and specialty metals such as stainless steel in the automotive industry. Weight savings, concomitant fuel efficiency increases and superior recyclability have driven the increased interest in aluminum in the automotive industry. Stainless steel is of great interest to the automotive industry because of its use in the construction of fuel cells.
Press forming of aluminum alloys and specialty steels has presented challenges, relative to low carbon steel, principally due to the very low strain rate hardening, low r (strain ratio) value and high galling tendency of such materials. Low carbon steels have significant strain rate sensitivity which is identifiable by a long arching stress-strain curve. Wrinkling, splits and other defects can occur in aluminum panels within the first 25% of the tool stroke using conventional forming techniques. Stainless steel and other specialty steels are also subject to cracking (breakage), and they can contribute to excessive tool wear when conventional forming techniques are employed.
To date, the use of non-symmetric forming has not been commercially feasible for most applications because the actuators have displayed minimal life. In most cases, actuators are capable of forming only one part before they fail. Actuators are relatively expensive to produce, and limited actuator life makes non-symmetric forming too costly.
The present invention provides a novel actuator and a method of non-symmetric forming that overcomes many of the disadvantages that are experienced using prior art forming techniques.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.